i have a list like this, let me put it back together and spruce it up with my name finally sleeping (kinda) refreshed brain. those twin club sales can be total life savers!!
best thing is think of it like renting something, you are going to sell most all of it back in 6 to 12 months, often for exactly the same price you bought it for (minus the commission the club keeps, thats the rental cost)
thinking that way i felt ok buying a bit over and figuring it out later.
also as far as mine goes, tiny clothing is what they have most of and the members usually already have grown out of it, so there is not so much struggle to find great clothing, rather go for the non clothing stuff on your list first, that stuff sells out faster as it is for more ages at times.
find out when the spring sale is usually, so you can think of what size they will be and what season it will be then and make sure you buy for everything till then and a extra month or so incase they move the sale, something that happens because of venue choices.
since your babes will be born near to the next sale focus on the important things and the great finds and assume there is always another sale!
i buy for anytime in the next year at each sale, i keep i little cheat sheet about what age/size my kids will be at each big clothing season or event. (beach =16-20m, camping = 18m, next sale= 22m, x-mas/birthday 24m, formal event = 27m hot weather)
once they were born i also kept measurements of them on me and a ruler, since clothing companies vary so much! so i know that for instance at the last sale her onesies were needing to be 15" and his 17" to fit, no matter what size they said.